1. Field of the Invention
The invention relates to actuating equipment for valves having a reciprocally moveable gate for blocking the flow path, such as gate valves, penstocks and the like.
2. Description of the Prior Art
Gate valves and penstocks have customarily been fitted with handwheels to turn screwed rods connected to the valve gate. Devices to render operation faster and/or less laborious, or to provide for remote operation are known, for example hydraulic actuators are disclosed in U.S. Pat. Nos. 3,226,078, 3,111,298 and 3,086,745.
Fluid driven devices are well suited to the operation of gate valves and penstocks since they generate reciprocating motion directly by means of a piston and cylinder. However, known devices utilise only a single fluid, either compressed gas or hydraulic oil and both fluids have disadvantages. Compressed gas piston and cylinders generally operate with an uncontrolled rapidity and are inherently unsuited to moving the gate of a valve or penstock over less than its full travel. Because of its resilience compressed gas would not hold a gate firmly.
Equipment handling hydraulic oil tends to be dirty and is messy to maintain--especially where dismantling is required--leading to greater maintenance costs and longer maintenance downtimes. Remote operation requires electrical equipment and runs of electric cable which may need to be protected from its environment or which may itself represent a hazard.
Electro-mechanical actuators remain in use, presumably because of the above mentioned or other disadvantages of fluid driven devices. One device which has been introduced (in the United Kingdom at least) provides an indication of the level of elaboration and complexity which is at present regarded as acceptable. This device has an electric motor to turn the screwed rod of the valve or penstock, but the motor is not fixedly mounted. Instead it is part of an assembly carried on the screwed rod. Movement of the assembly is opposed by springs which are sufficient to restrain it against the motor torque only as long as the gate of the valve or penstock continues to move. When the gate reaches the end of its travel the motor assembly moves on the screwed rod, and its motion is detected by limit switches which turn off the motor. Should a limit switch fail to function the motor assembly would screw itself off the screwed rod, or stall when it reaches an obstruction to its further movement with concomitant overloading of the motor.
Subsequent to making the present invention the applicant has become aware of proposals for equipment which supplies compressed gas to an oil reservoir to pressurize hydraulic oil to the pressure of the gas. None of these proposals, however, has anything to do with the actuation of gate valves or penstocks; they occur in other arts. U.K. Specification Nos. 1,202,913 and 1,321,291 relate to landing gear for vehicle semitrailers; U.K. Specification No. 1,392,418 is concerned with apparatus for moving an X-ray generator; U.S. Pat. Nos. 2,573,993 and 2,403,912 relate to hydraulic presses, in the latter specification there is disclosure of using compressed air to raise oil to the air pressure to move the press and subsequently to an enhanced pressure to complete the press stroke, and U.S. Pat. No. 3,370,824 describes a ladle for molten metal which carries a supply of compressed gas to drive a hydraulic oil pump for actuating a stopper rod constituting a valve for the molten metal.